The packaging industry is continually looking for ways to reduce the amount of metal used in the package while improving or maintaining the integrity and functionality of the package. This is of particular importance in the area of beverage containers due to extremely high volumes. With such large volumes, small reductions in the materials used for each package add up to a very significant savings of money and of metal resources.
One area where a great deal of work has been done to reduce material costs and improve strength is the end wall which closes a conventional, generally cylindrical metal beverage container. As is well known, this end wall, or container end, is less able to withstand internal pressurization of the container than the sidewall for a given thickness of metal. Thus, for example, while the industry has been able to reduce the sidewall of a two-piece aluminum beverage container to about 0.004" in thickness, the container end is on the order of 0.011" to 0.012", depending on the intended purpose and design of a container end. Reduction in the thickness or "gauge" of a container end for a beverage container of a few thousandths of an inch will result in large raw material savings. Because simple reduction of gauge causes an end which may not meet industry standard strength requirements, improving the strength of such container ends allows reduction while meeting industry requirements.
The container end typically has a center panel surrounded by a countersink which is integrally connected to a peripheral flange or curl. The curl is provided to double-seam the container end to the container. Internal pressurization of the container can cause the center panel on the container end to dome, or bulge, upwardly due to axial upward forces. In turn, the axial upward forces acting on the center panel cause radially inward forces on portions of the countersink which may pull it away from the container sidewall allowing the center panel to bulge even higher. A variety of problems are encountered if the center panel rises above the double seam of the container. Historically, this has been compensated for by utilizing a relatively thick container end. However, in order to thin, or downgauge, the container end, an improved container end design was needed in order to help the container end withstand bulging and buckling forces.
Considerable work has been done to improve the buckle strength of a container end through modification of the countersink area, usually in concert with other structural elements of the container end. The conventional practice in making a container end today is to start with a shell that includes a countersink portion between the center panel and the curl. The countersink includes an inner wall and an outer wall joined by a countersink bottom. Typically, the countersink bottom of the shell has a relatively large radius. The inner wall is joined to the center panel by a curved shoulder. The shell is made in a shell press for converting a disk of metal, or cutedge, into a shell. The shell is then processed in a conversion press, where the shell undergoes various operations to be converted to a finished container end. For example, a ring pull or non-detachable tab is attached to the end, and scorelines defining a pour opening panel are provided for a pour hole. A container end maker may purchase standard shells from a vendor or operate its own shell presses.
The structural design of a container end can be advantageously used to reduce the material required to produce the container end. Improved strength resulting from an improved structural design will compensate the container end for loss of strength due to reduction in gauge thickness.
One such design consideration believed to provide additional strength to the container end is to have a small radius (i.e., a tight bend) in the countersink portion of the container end. However, due to the current gauge thickness presently used to form container ends, it is difficult to achieve the desired countersink configuration without thinning or ripping the metal of the container end.
One method of forming a container end of low gauge thickness having a tight countersink radius is disclosed in a co-pending application Ser. No. 07/955,921, filed Oct. 2, 1992, now U.S. Pat. No. 5,356,256. In that Application, a method of reforming a container end to have a single tight countersink radius is disclosed. The countersink portion is reformed progressively in several steps so as to not place undo stress on the metal. However, forming a single tight radius in the countersink bottom has the effect of bringing the inner wall extremely close to the outer wall. This makes it difficult to attach such ends to container bodies using industry standard tooling. Accordingly, a need exists for providing a container end having a countersink bottom with at least a portion having a tight bend, or radius, which can be easily secured to a container body using industry standard tooling.
Another concern associated with low gauge ends is to direct any potential buckling away from certain portions of the container end. As mentioned, the center panel typically includes scorelines which define a pour opening panel. Also, a non-detachable tab is secured to the center panel by a rivet. The tab is pivotally mounted on the rivet so that upward movement of a portion of the tab causes an opposing portion to engage the pour opening panel and break or rupture it along the scorelines to open the pour hole. In recent years, container ends have been made with stay-on tabs and non-detachable pour panels in which the scorelines do not completely surround the pour panel. Thus, a portion of the pour panel remains secured to the center panel after the scoreline is ruptured.
When secured to a container, the center panel of the container end, including the tab, is positioned below the double seam, or "chime," of the container. As the end wall is downgauged, it becomes increasingly vulnerable to a variety of problems resulting from internal pressurization of the container. For instance, the doming problems discussed may lead to undesired openings or scoreline fatigue. Scoreline fatigue can result in leaking, or in more severe cases, the pour panel blowing off the container end and effectively becoming an airborne missile. Additionally, the container end may experience localized buckling, whereby a portion of the container end, typically in the countersink, is deformed axially upwardly above the chime. Localized buckling proximate the pour opening panel can also lead to pour panel blow-off or scoreline fatigue.
As is well known in the art, forming an annular band of reduced thickness along 360.degree. of the shoulder of the center panel provides additional resistance to buckling. This is sometimes referred to in the industry as "coining" the panel shoulder.
U.S. Pat. No. 4,503,989 (Brown et al.) discloses one method of directing potential buckling in a container end. Brown et al. discloses a container end which includes a non-detachable pour opening panel defined by a non-continuous scoreline of reduced residual and a hinge portion located proximate the center of the center panel of the container end. The pour opening panel extends from the hinge portion radially outward towards the panel radius and terminates in a pour opening panel nose. A tab in the form of a pull ring associated with detachable pour opening panels is asymmetrically secured to the pour opening panel by a rivet positioned proximate the, pour panel nose and spaced only slightly from the panel shoulder such that the tab and rivet are asymmetrically located on the center panel of the container end. The tab and pour opening panel cooperate in a manner so that upon rupturing of the scoreline, the pour opening panel is pulled upward exposing the non-public side of the pour opening panel.
Brown et al. further discloses a method of pivoting a lifting portion of the tab downwardly. A region of the center panel radially outward from the rivet and extending to the panel shoulder is coined, thereby providing loose metal and permitting the coined region to rise slightly due to internal pressure in the container. The upward movement of the coined region tends to lift the radially outward portion of the tab and pivot the lifting end of the tab downward.
Additionally, Brown et al. discloses coining a segment of the panel shoulder less than 360.degree. centered around the nose of the pour opening panel to direct potential buckling away from the reduced residual portion of the scoreline and thereby reduce fatigue on the scoreline in the instance where buckling has occurred. The coined region radially outward of the tab and the coined segment of the panel radius overlap so that there is no uncoined portion between the coined panel radius segment and the coined region.
However, by directing potential buckling in the manner described, Brown et al. cannot derive the benefits of a full 360.degree. coining of the panel shoulder while maintaining the ability to direct buckling away from the pour opening panel. Furthermore, Brown does not disclose a container end having a reformed countersink segment to provide such direction to potential buckling.
The present invention is provided to solve the above problems and concerns as well as other problems.